text:
Does a tandem ultralight need flaps on the rear wing if the front wing has flaps?
Does a tandem ultralight need flaps on the rear wing if the front wing has flaps?
I understand that most Rutan canards don't have flaps as I was told if the main wing has flaps, the canards need one too.
Rutan did make one with flaps on both the canard and the main wing.  I think it was called the Grizzly.
So... what about a tandem aircraft (with positive lift on the horizontal tail) with flaps on the front wing ? Does the rear wing need flaps too?
Can't you just use the horizontal trim tab as a flap if you need more rear lift to offset extra lift of the main wing with flaps extended?
After all, wouldn't most conventional aircraft with rear cg and slow flight ( low Cm) in a dive with a symmetrical horizontal tail, with elevator down, be generating positive tail lift anyways, so they are acting like a tandem aircraft, no?

The horizontal tail on a normal airplane never generates positive lift in a static condition, only in transient ones, like a hard push over.  Once stabilized in a dive, the tail is back to making down force.  The aft limit of the CG must always be forward of the neutral point so when not maneuvering there is a positive pitching moment and the tail is still generating down force whether level, diving or climbing.  Otherwise, the airplane becomes unstable in pitch.

Little off topic, but I think there is a difference between descending and diving. Descending is cutting power, so less lift, so the plane descends.  I defend a dive  with forward stick, so a foil like a naca012 would have a positive aoa, therefore it would generate positive lift.  Is this correct?

When you push the stick forward you are reducing tail downforce, the airplane pitches over because its pitching moment wants to do that, and it accelerates until the original tail downforce is restored with the new forward stick position you set.  There is still a net downward lifting component, making the same downforce as before, but you are going a lot faster so you overall negative AOA needed is way less. The horizontal tail is like a wing with a flap (the elevator) where the flap is reflexed up (but flipped over). The reflexed wing is still making lift, but it needs to go faster to do it.

@JohnK: No, the tail doesn't need to fly at a negative angle of attack (generate downforce) in order to have static stability in pitch.  It merely needs to fly at a lower angle of attack than the main wing.  The OP's intuition is correct on that point.

@JohnK: So all Airbus aircraft since the A310 are not normal. What then must they be called? Abnormal?

@PeterKämpf are you saying Airbuses can be loaded with the CG aft of the neutral point so the tail has to lift?

@JohnK Are you still believing in that nonsense? The tail may very well have lift while the CG is still ahead of the NP. Just less per area than the wing.

A word of caution to small GA plane designers, an aircraft that is not STATICLY stable will very easily stall (not good) on approach to landing if the pilot takes eye off of airspeed even for a few seconds.  This is why static stability should not be lightly discarded (before at least understanding it), even though it is theoretically possible to have the rear "wing" positively lifting.  The high wing Piper Cub, Fiesler Storch, Cessna   172 designs represent the hard earned lessons of the first 40 YEARS of aviation.  Study of them is Aviation 101.

@PeterKämpf a tail lifting UP while in steady state trimmed flight?  What opposes the ND pitching moment of the CG acting about the neutral point then?  I'm not talking about gust or maneuvering cases where loads on the tail are momentarily reversed.

@JohnK: Are you confusing neutral point (NP) and center of pressure (CP)? All you need is to have the CP in the same station as the CG - this is very well possible with lift on the tail. The NP is still further aft, so the plane is stable.

@PeterKämpf IIRC, NP is the longitudinal point at which pitching moments of the entire body is zero. I think of NP as the "effective" CP, or, CP adjusted for other pitching moments that have influence, like the fuselage surface area, offset thrust line etc.  Everything I've ever read indicates that static margin requires the CG to always be forward of the NP, so there is always a net nose down pitching moment, outside of transient maneuvering or gust cases.This means the tail has to be providing down lift in steady state conditions to provide good static stability.

@JohnK -- Re "The horizontal tail on a normal airplane never generates positive lift in a static condition, only in transient ones, like a hard push over. Once stabilized in a dive, the tail is back to making down force. " -- for a contrary view, see https://www.av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium

@JohnK -- in more detail-- "I took the same Skyhawk and put a small pilot in the front seat, a moderately large mad scientist in the back seat, and 120 pounds of luggage in the rear cargo area. That put the center of mass right at the rear of the envelope, so the tail had to produce considerable positive lift in order to maintain equilibrium. The airplane still had plenty of stability. (As far as the pilot could tell, it was just as stable as it ever was.)"

@JohnK -- continuing-- "The easiest way to determine whether the tail lift is positive or negative is to observe the direction of motion of the tip vortices, as discussed in section 3.14. To observe the vortices, I attached a streamer of yarn, about half a yard long, to each tip of the horizontal tail, at the trailing edge. The streamer gets caught in the vortex, so its unattached end flops around in a circle. When the tail is producing positive lift, the circular motion is in the direction shown by the green “circulation” arrows in figure 3.29, i.e. downward on the inboard side."

I thought the whole reason why classic config will always be more efficient than a canard or tandem  is that they fly with zero lift on the tail, which is when the Cm is o.  So in a straight and level  flight at the optimum point, there is no downforce of the tail, no?

@quietflyer I'm not sure I buy his thesis. If the tail was making positive lift, the load on the elevator itself, which wants to trail when hands free were it not for servo force of the trim tab, would be up, and the trim tab's servo force would have go the opposite way to counteract it, applying a down force at the elevator TE, not an up force.  I've never seen a trim tab work backwards like that.  Certainly doesn't on my airplane.

@JohnK  so you are saying that we can observe that a trim tab always applies an upforce to the trailing edge of an elevator, and this can be taken as evidence that the horizontal tail as a whole (including elevator) is creating a down force.  Hmm.  There could be a question for Aviation Stack Exchange here--.

As an interesting thought experiment, think through the action of the anti-servo tab on an airplane with a stabilator tail, like a Cardinal or Cherokee, where all of the static stability comes through the anti-servo's action to drive the surface to a given incidence to hold a trim speed, and think through how that tab could function if the tail was lifting up instead of down.  It's mechanically geared to only work with a down lifting surface. I think this issue has been beaten to death many times on here and there is unlikely to be a resolution that satisfies everybody for some time to come ;)

Let's beat it some more.  Take a tandem with 2 identical wings, mount the fore wing at 5 degrees greater incidence, where do we put the CG?  It must be a little closer to the fore wing, which is at a higher AOA on the Coefficient of Lift Curve (it will also stall first).  Theoretically all well and good.  Someone named Langley tried this around 120 years ago.  Issue with tandems (and biplanes) is wing interference.  Even if the rear "wing is a much smaller flat plate with downforce, the design remains a "tandem" and interference remains an important design issue.

Yes @Fred, that is right.

Would there still be interference if the gap and stagger is 2 chord?  I heard at that gap and stagger, the 2 wings act independantly.  Is that true?

@JohnK Centre of all lift must be behind CG. Stabiliser down force still has good reasons

@Koyovis yes CG must be forward of the neutral point, and as a result the tail always lifting down, except in transient states.  That's what I've been arguing.

@JohnK Please check the links

As the love of tandems continues I will answer.  Because you are balancing lifting duties between two wings and (hopefully) still have the one in front stalling before the one in the rear, any configuration change in the front should be proportionately matched in the back so that the stall characteristics remain safe when flaps are deployed.
Deploying flaps increases the Angle of Attack of the wing and may cause a shift in the center of pressure.  So the design needs to be approached with care.
You could also put slats and flaps on both wings of a biplane too, but the whole point is to have one (smaller/less drag) wing for faster cruising flight and a larger, higher lifting wing for low speed flight (take off/landing).
For a GA personal use recreational aircraft, you may not need flaps at all.


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Does the DC-3 have a prop sync?
I'd like to ask if the DC-3 has a prop sync.
I found nothing about prop sync in the DC-3 operations manual, in the running specs, design features, or in the chapters about its engine and propeller. I've read the Wikipedia article about prop sync (which  didn't mention it) and this What is propeller / engine sync and how does it work? (which was great, but didn't speak of the DC-3). I've used this last link (which highlights the instrument) to check pictures of the cockpit and didn't see anything that looked exactly the same, but I don't think the instruments would look exactly the same in aircraft of different manufacturers (Junkers and Douglas). I assumed it didn't, but being that it's a twin-engine (which qualifies as a multi-engine) turboprop aircraft I'd have thought it'd make sense to have one. Can anyone help?
Thanks

The DC-3 has radial engines, the BT-67 is the turboprop version.

Judging from this video, it does not have an automatic prop sync. When throttles are advanced you can hear the harmonic beat of the twin propeller blades, and during the climb the instructor seems to manually set the levers to reduce the harmonic set the correct manifold pressure. He may or may not have an indicator to help him, but judging from the sound effects there is no automatic synching taking place.
And here is an old C-47 cockpit procedure training film, on how to fly the aircraft. Nothing on synchronising the propellers.
That beautiful engine sound is not produced by a turbine by the way.

Apologies for my ignorance. I thought propeller-driven meant the same as turboprop. I don't really understand much about airplanes. I'm trying my best though. it's all very conceptual when you are not a pilot or  a mechanic .  I've learnt about rules and lingo used by FAA and EASA and read plenty of manuals, but I never figured something apparently clear could have meant something so different. In my head anything that had a propeller would be a turboprop. I realise how daft that sounds but it's the truth anyway. Thanks for helping me out.

No worries, glad to be of help, that's what we're here for.

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text:
Can I change the max bank angle on the Garmin G1000 in the C172SP?
I wonder if I can change the max bank angle on the G1000/GFC700 avionics in a C172SP, like you can on the MCP of the 737NG?

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text:
I have bad eyesight, can I still be a commercial pilot in Malaysia?
So, I have really bad eyesight (short-sighted): without my glasses, I can't read text unless it's near. So can I be a commercial pilot?

What is your corrected vision? (with your glasses)

According to the Civil Aviation Authority of Malaysia's medical standards:

(c) Distant visual acuity, with or without correction, shall be:
  (1)
  in the case of Class 1 medical certificates, 6/9 (0,7) or better in
  each eye separately and visual acuity with both eyes shall be 6/6
  (1,0) or better;
  (2) in the case of Class 2 medical certificates, 6/12
  (0,5) or better in each eye separately and visual acuity with both
  eyes shall be 6/9 (0,7) or better. An applicant with substandard
  vision in one eye may be assessed as fit in consultation with the DCA
  subject to satisfactory ophthalmic assessment;
  (3) applicants for an
  initial Class 1 medical certificate with substandard vision in one eye
  shall be assessed as unfit. At revalidation, applicants with acquired
  substandard vision in one eye shall be referred to the DCA and may be
  assessed as fit if it is unlikely to interfere with safe exercise of
  the license held.

The key words appear to be "with or without correction", i.e. it's completely fine if you wear glasses to correct your vision. That seems to be standard around the world, e.g. the US standards also allow correction.

I am in Malaysia as well.
Your visibility problems may not stop you from getting a CPL/multi engine/IR license but once you've got that in hand you need to pass thru the airline
interview.
Currently the job market for pilots in Malaysia is a bit soft in that there are more pilots than jobs, as such airlines can pick and choose. If you have two guys with the same license you will definitely choose the one with better health, better academics etc.
Not trying to kill your dreams but this is something you really need to think about before you pursue a commercial license.

yes, I know, I was just wondering. I actually only plan to get a PPL, just wanted to ask about eyesight so that I know if I want to pursue a professional career in Malaysia.

This depends on your jurisdiction but the FAA will allow it so long as your eyesight can be corrected (i.e. with glasses) to 20/40. You can find the full vision requirement here.

Near Vision:
20/40 or better in each eye separately (Snellen equivalent), with or
  without correction, as measured at 16 inches

So I still need 20/40 eyesight?

And I am in Malaysia

@lpydawa 20/40 corrected (with glasses). I dont know the Malaysian regulations but you could always train in the states.

Thanks, by states you mean the USA right?

@lpydawa yes the USA

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How much physical strength is required to control a Cessna 172?
I have heard a number of folks tell me that one needs a certain amount of physical strength to be able to control the Cessna 172. Is that true? I'm flying sims now, just wanted to know.

The way you asked it is very hard to answer. It's way easier to answer if you you ask "how much lbs of force each input requires". For example, will you say one needs strength to operate the parking brake on a car? About half the people say yes while the other half say no!

Ok, noted, thanks.

I feel as if there is a dupe somewhere on this site but I can't seem find it at the moment.

FWIW, I never had a student, male or female that hard trouble with the elevator forces of a 172. I did observe smaller females who had a little trouble with the elevator forces of a 182, and I had some of both sexes (myself included) that found the elevators forces of a 206 a bit much.

I will take this from a more general perspective.
A Cessna 172 is part of the CS-23 category (in EASA land, see the equivalent Part 23 for FAA land).
In the relevant document, point CS 23.143, we find the maximum forces a pilot might be required to exert on the control columns so that the aircraft can be certified.
As the Cessna 172 is certified, we then know that it respects these requirements, so the maximum forces required to pilot a Cessna 172 (and any other CS-23 aircraft) are:
\begin{array}{l|c|c|c|}
\textrm{Values in Newton applied} & & & \\
\textrm{to the relevant control} & \textrm{Pitch} & \textrm{Roll} & \textrm{Yaw}\\
\hline
\ \ \ \ \textrm{For temporary application –} & & & \\
\textrm{Stick} & 267 \textrm{ N} & 133 \textrm{ N} & \\
\textrm{Wheel (two hands on rim)} & 334 \textrm{ N} & 222 \textrm{ N} & \\
\textrm{Wheel (one hand on rim)} & 222 \textrm{ N} & 111 \textrm{ N} & \\
\textrm{Rudder pedal} & & & 667 \textrm{ N} \\
\hline
\ \ \ \ \textrm{For prolonged application –} & 44.5\textrm{ N} & 22\textrm{ N} & 89\textrm{ N} \\
\end{array}

IIRC there is also a variant with manual flaps that are actuated with a bar in the cockpit, that might also require a bit of strength.

Might be useful to add some comparisons as context to this answer - 267 N max force (for pitch+stick) is the equivelent of the force exerted by gravity on a 27Kg object. This sounds like an awful lot to be exerting with your arms...

@stripybadger that's for a temporary application. I'm sure you would be able to pull/push 27kgs for a brief amount of time. You can see that the prolonged case has much lower limits (about a fifth, or 5kgs)

I'd also like to see that put into perspective (for example, is 27kg like a car steering wheel in pre-servo times? Or how does it compare to a modern car with servo?). There are also angles for the rotating pieces to take into account. I'm pretty sure there are people who cannot, even momentarily, lift a 27kg item, or at least intuition tells me so. Considering the final fact that these are absolute maximums of the class of plane in question, the actual Cessna could be anywhere else on the spectrum...

@AnoE I'm sorry, but not having data means I am not going to compare it to anything, I am not going to invent data for the sake of internet points. Also, it is not lift, but push/pull. As for the Cessna being somwhere below that limit, yes, but I do not work for the manufacturer, and I do not have the manuals needed to have the actual numbers for that model. If you can find them feel free to answer.

It depends on what you mean by "strength". In general, it is "not much".
I've flown a lot of sims and also flown a real Cessna 172. It is true that most sim controllers (especially joysticks) require almost no effort to move at all. In that perspective, then yes, flying a real airplane takes some strength, especially if you compare the force required to achieve max deflection (not recommend in flight!!!).
The flight controls of a light aircraft are still light enough that a healthy adult can manipulate them very easily. In fact, the correct technique is to use two or three fingers on the yoke. Most first-time students grab the yoke firmly in their hands, this is wrong because you cannot feel the airplane. If one has the strength to drive a modern-day road car, one can certainly fly a light airplane.
I have also spent some time in a B737 training simulator (not PC simulators - I mean the real sims used for training). I was surprised to find the controls much heavier. I would describe the feeling as solid and fluid. They are still easy enough to manipulate such that my mind can focus on elsewhere instead of the physical movements of the control.

My primary flight instructor always told me: if you're using more than two fingers, you're not flying the airplane correctly.

I've never flown a 172, but I fly a Piper Warrior which is in the same ballpark. You definitely need to apply a lot more force than with a flight sim yoke and provide positive pressure (Remember, you're fighting the airflow ultimately), but you don't need to be "strong".
I'd imagine anyone who can drive a car would have the strength to fly a plane. Remember, most flying is one handed and most pilots will use their less-dominant hand (Left hand) so it can't be that bad! I do find the Warrior needs a lot of backpressure to rotate and flare.
I've struggled to find any mathematics to add to this, but I've found a few quotes which imply you need approximately 1lb of backpressure to hold the plane level for every 6kts without trim (The whole point of trim is to avoid this and make life easier)
So, at 100kts that's 16lbs / 7kgs.

Thanks for the answer, do you thinknits the same for the 737NG?

Wouldn't have a clue I'm afraid - those aircraft have hydraulic assist and all sorts of fun little things to help. The 172 and Warrior are simply Yoke -> Cable -> Control Surface

I see, thank you.

So, lets say Im cruising in a Cessna 172, at 122 knots, that means I need to wrestle 9kg. That must be unpleasent.

@lpydawa Only if you don't trim - I have no idea how accurate it is because I've got no way of measuring or even estimating what an amount of back pressure is. But yes, flying a wildly out of trim aircraft can be very hard work and a bit wrestle-like

Yes, thanks for your answer.

@lpydawa Don't pin me on this but I think it's similar in a Boeing 737. However, as Dan said the flight controls on the 737 are hydraulically assisted. But IIRC you will still feel the different forces at different speeds because it's still a direct control, just assisted. In an Airbus however you will feel nothing, because it's controlled by the Fly-by-Wire system.

@Dan: You could get an idea if you pull the yoke with one of those luggage weigh scales. Put one hook around the yoke and your finger through the other one. (I'm not a pilot so ...)

@ipydawa actually it is not unpleasant at all. At such a high speed the control surfaces only need to deflect a tiny bit to have a big effect. A light gentle touch is all that's needed at any speed really - anything more and your technique is wrong.

@Dan: OTOH, flying one that is properly trimmed can be done with the pressure of one finger.

@jamesqf are you sure its that easy to control the plane?

@lpydawa Not a '172, but during one instructor takeover during a landing in the twoseater I fly, the instructor inadvertantly hit the trim controls (which are really sensitive touchbuttons on top of the shared control column) and trimmed to one extreme. (I honestly don't recall if it was full nose up or full nose down trim.) When I got the controls back, my first reaction was "jeez, this is heavy!", but as we were really close to the runway at that point I didn't look into it. The airplane was still perfectly flyable, and I'm not what anyone would consider very strong. I retrimmed soon after.

@lpydawa As for flying a properly trimmed airplane, that really is lightweight. Properly trimmed, even an airplane without an actual autopilot will basically fly itself in reasonably calm air. It really only gets heavy when you try to fight the aerodynamics.

@lpydawa: Yes.  Though most of my time is in a Piper Cherokee, I've spent a couple of dozen hours in 172s.  Get the plane trimmed properly, and you just have to sort of suggest where you want it to go :-)

I taught people of all sizes to fly for about 30 years. I taught my commercial students how to take off and land without even touching the yoke. So it is possible to control a c- 172 with no yoke force whatsoever. Just rudder, throttle and trim. So by the use of proper trim use, one can control the aircraft using the yoke with only ounces of force. Don't let people discourage you.

I really wouldn't encourage flying with just the trim. As an exercise, yes; it's a good backup, since in many airplanes elevator trim is separate from much else. As SOP, not really; the trim responds too slowly for that. What happens if you need to execute a go-around and don't have your hands on the control column? Sure, it might be quick to move them there, but why should you need to? Even when coming in at near touchdown speed, a second is a long time...

The amount of physical exertion required while flying a 172 depends largely on what you're trying to accomplish; for example, cruising in a trimmed attitude requires almost no effort whatsoever. However, purposely inducing an aggravated stall/spin action then recovering requires considerably more effort.
Since recovering from all situations is important while flying as a PIC, one should give it a try before pursuing training in earnest.
Personally, the most exertion required is climbing up to manually check the fuel levels in the wing tanks. But once I got the hang of it, it became quite easy.

I have several hundred hours in a 172, while it's a bit heavier on the controls than a 152   it isn't much. If you have normal movement then you can likely fly a 172. A benefit to control force in real aircraft vs Sims is the feedback you get through the controls with renders Sims largely ineffective in many cases. OK for some stuff mostly instrument related but it will never replace actual flying. The 172 was a lot easier to fly than the turbine modified Grumman Goose I flew, that thing was like trying to fly a dump truck with manual steering.

I fly both Cessna and piper aircraft.  The flight controls on a 172 require a lot less force.  The flight controls on a piper feel much heaver..you feel like your flying a much bigger airplane. So to answer your question it does not take a lot of strength to fly one.


text:
Did British aircraft fly at a specific speed while returning from missions during WWII?
I remember reading a whole lot ago (so I am currently unable to find the reference, sorry) that during WWII, British aircraft would return from missions at a specific speed that was unattainable from German aircraft, i.e. it was either too fast for some, or too slow for others.
This had the immediate benefit of enabling radar operators to distinguish friends from foes without the need of any additional equipment or method, they would simply compute the aircraft speed and determine if the aircraft was British or German.
Is it true? What was that speed?
Why wasn't there a German aircraft capable of flying at that speed?

I recall reading something similar: designated location/course/speed/altitude combinations that were given to RAF pilots secretly and which served as a "Don't shoot me" to radar and AAA gunners. If I can find a reference I'll post it up.

British pilots could squawk their parrot, the ancestor of IFF.

I was a light attack jet pilot aboard the USS Nimitz during the 1980's. I did 2 cruises in the Mediterranean and was airborne during the initial shoot down of the Libyan MIG's. We also spent time off Beirut after Sadat was assasinated. We came close to initiating strikes against one country during my tenure. I say this just to make the point that I have operational experience.
We had procedures for coming back to the ship during times of threat. They included flight paths for approaching the carrier. If you were obeying the procedures you would be identified as a friendly aircraft. Airspeeds were a part of the flight path, but they had little to do with the capabilities of adversarial aircraft. I do remember the airspeeds were slow making our aircraft vulnerable to attack. My guess is that they would engage the threat separately.

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text:
Are there any footwear regulations for private pilots?
Just wondering, as I'm working on my PPL during a hot summer, if it's legal (but probably more importantly, a good idea) to wear flip flops to fly a plane? I'm training in a 172, if that's an important factor.

No regulation I'm aware of, but flip-flops are probably not the best. If there is an underpanel fuel (Primer) or oil leak (pressure guage) then that's going to hit bare skin and could irritate, or worse, burn the bare skin.  If you need to get out in a hurry, you don't want something dangling that could get caught.  Or fall off as you step out onto hot pavement. Or dangling around as you climb a ladder to refuel the plane.

I would worry about the flip flops falling off inadvertently and possibly interfering with something or being hard to retrieve, but, for what it's worth, in hot weather, I often flew 172s and 182s barefoot back in the 1980s.

There is no policy on the books that I can find in regards to this. Loosely if flip-flops make it hard for you to operate the rudder it would fall under the catch all:

§ 91.13 Careless or reckless operation.
(a)Aircraft operations for the
  purpose of air navigation. No person may operate an aircraft in a
  careless or reckless manner so as to endanger the life or property of
  another.

However I was at an FAA WINGS lecture not all to long ago on preparing for accidents. My local FSDO's advice was to "dress for the crash" i.e. if you plan to fly over snowy terrain consider wearing boots etc. With this in mind flip-flops are more than likely not a great choice. The need to walk considerable distances over terrain is not unheard of after an accident.

That's a super sad story, but you're right, definitely something to be learned there.

Even for driving a car, you can be cited (typically after an accident) for "negligent driving" for driving with flip-flops or barefoot.  "Negligent driving" is a legal way of saying "you should have known better."

Remarkably then, easily removable flipflops - or even bare feet! - could be called for over water, I guess!

Flying in tropical areas (outside the US), I have frequently observed the locals going barefoot.  There is no dress code in 14 CFR 61, 91.
Large boots can be problematic.  I tend to wear sneakers, even when flying for work.  I get better feel of the rudder pedals, and boots and dress shoes are hard for me to find, as I have very large feet, and they tend to have even larger "foot prints" because of the design of the footgear.
I can tell you that more than one student who showed up in pumps had problems with rudder and brake operation.  Usually it only took one flight to convince them to bring slides or some other shoe.


text:
Has anyone considered filling aircraft voids with helium to reduce the risk of fire?
Watching air crash investigations, and aware of problems with batteries, the issue of fire and sparking is recurrent. As a noble gas, pure helium would not burn, and if I understand correctly would reduce though not necessarily eliminate sparks.
While any additional lift would be marginal, it would likely be sufficient to compensate for the materials necessary to ensure a sufficiently sealed envelope.
So... Has anyone considered filling aircraft voids with helium?

Helium is expensive and limited. As the 2nd smallest element, it is very good at leaking out of any space that isn't 100% sealed (air-tight is not good enough, "helium-tight" is much tighter).

Also the dielectric strength of helium is less than that of air (if I'm reading the table right) so sparks would be even more likely

Consider replacing "helium" with "helium or nitrogen" in your question. Nitrogen is much more abundant than helium, so nitrogen is usable in a lot of cases where helium is not.

@TannerSwett's right, also N2 will escape much more slowly (a much bigger molecule, much smaller difference in partial pressures)

@abelenky Cost isn't an issue, this is a news-driven myth for He gas users. Volumetrically, gaseous helium costs less than jet fuel. The people who complain are those who use liquid He for cryogenics. Like any cryogenic gas, the expansion ratio is huge, 1:757 for He. Considering boil-off and liqufication costs, you have to pay $1000 of LHe to fill the same amount of space $1 of He gas does. The leak rate is the issue.

Who needs helium-tight? Argon-tight would accomplish the same goals. Shoot, nitrogen-tight would also work, or simply nitrogen-positive-pressured like the body fuel tanks.  Nitrogen is even a very slight lifting gas.

Do battery fires even depend on oxygen from the air?  Or are they just the battery chemicals reacting with each other?

@user71659: Nitrogen is still cheaper, though, even if the difference doesn't add up to very much compared to the total operation costs of the aircraft.

@abelenky: he tight is almost impossible, it even diffuses through metal

@abelenky: "Second-smallest element" is true but misleading, because hydrogen does not occur in elemental form. "Smallest molecule" is a lot more telling.

Helium is a limited resource and has been in short supply for some time (customers on allocation). Many facilities are now recovering the gas. Nitrogen or Argon would work. See, for example, this article. It's a pain to have to work around it.

Why Helium specifically? Burning is reaction with oxygen, so as long as you displace oxygen, it does not make that much difference what you displace it with.

@jamesqf My understanding is that lithium-based battery chemistries support their own combustion, so once the process gets underway, just removing the oxygen doesn't help much, and oxygen isn't required for starting the fire; this is part of the reason why such fires are such a big problem when they happen. This is in contrast to many other battery chemistries, in which fires can be extinguished simply by removing the oxygen.

Others have said it, but it really cannot be stressed enough. Helium is an insanely valuable non-renewable resource with a very limited supply. I still think it is absolutely criminal that people waste it for absurd things like party balloons. When it runs out we have no means to make more other than waiting millions of years for the Earth's radioactivity to produce more or making it in some sort of nuclear reactor. There are scarce few applications where you really really need helium and can't use anything else. Unless it's one of those cases, and it really, really, important, forget it.

@JanHudec ok, let's displace it with fluorine...

I was expecting the title to end in the somewhat more silly question "Has anyone considered filling aircraft voids with helium to increase lift?"

Helium has about 1g  of lifting force per liter (at STP), so if you fill an empty ULD-3 cargo container (4.5m^3=4500 liters) with helium,  it'd be around 10 lbs lighter. Though I'd bet that the seals to make it helium-tight (which is harder than just "air-tight) would weigh more than that. And at higher altitude, air is less dense, so the lifting force is less, you'd have to release (or reclaim) some of the helium to optimize the buoyancy.

Yes, it has been considered, but with some differences.
First, the inerting gas used is not helium - it's nitrogen. Nitrogen also doesn't support combustion, and is for practical purposes inert. It also has major advantages over helium in this role. For one, nitrogen is very close to air in density, and doesn't leak as easily as helium, so it's easier to retain.
Nitrogen is also much easier to obtain by separating air. It can be done on the ground with nitrogen carried in cylinders, or generated onboard with OBIGGS. If you were to use a more exotic gas, it would be halon, which also breaks the chemical reaction sustaining most fires.
In practice, inerting is only applied to fuel tanks, not to avionics compartments (which house the batteries). One reason is that inerting won't prevent lithium fires: the batteries already contain all the reagents they need.
Nitrogen would prevent a secondary fire, but another reason comes in play: avionics compartments aren't sealed. Fires in these bays, like in the cabin, can be addressed by the crew. This requires insulation for the lithium cells (as has been installed) and fire detection in the compartment to allow response before the secondary fire is out of control.
But, as the first linked paper states, it's certainly possible to improve on that with quick automated fire suppression. Aircraft safety design is a set of tradeoffs and it's an evolving field. It's possible that in some years, as the amount and importance of avionics inevitably increase, more weight will be spent on their fire protection.

Military fighters do use Halon. Also notable are developmental systems to break down the fuel vapors into CO2.

@user71659: that link has lots of great info!  F16 uses halon, but more modern planes like F35 and some civilian jets use air-separation modules to split bleed air into nitrogen-rich vs. oxygen-rich streams.  (Suppressing fire in fuel tanks doesn't require zero oxygen, just low enough that fuel vapour won't combust or detonate.)  A tank of Halon replaced a tank of LN2 that had to be refilled before every flight.  But ASM replaced both of those by generating it on the fly.

Also note that depriving crew accessible areas of oxygen is a hazard. Sooner or later, there will be an error and someone will make their way into such a compartment before normal atmosphere has been restored. Even worse if the low-oxygen atmosphere starts to leak into and replace the passenger cabin air...

AIUI the cabin floor of airliners is NOT a pressure bulkhead. So there have to be vents between cargo hold and passenger cabin to protect the floor in the case of sudden decompression..

@PeterGreen Yes, those vents are in the side walls behind the side wall liners. At least on Boeing aircraft, the floor does not extend all the way to the skin of the airplane, it stops on the inside edge of the bulkhead, leaving about a 3 inch gap. Some, but not all, of the sidewall liners themselves may have a visible grid of holes near floor level, which improves the air path through this gap.


text:
When calculating the lift coefficient, what area is used?

Lets say I had this rectangular double curve wing. Orange represents the chord length, red represents the airfoil outline length below the chord line, green represents the airfoil outline length above the chord line, and blue represents the wing span.
If I were to calculate its coefficient of lift, I would need to know the area. Which area does it refer to? Could you explain how to calculate the desired area using this diagram?
Could it be the length of green multiplied by the length of blue? Could it be the length of green and red multiplied by the length of blue? Or something else?
Correct me if I am wrong, but in this case, I think the planform area would be the length of orange multiplied by the length of blue.

Reference area for the aerodynamic coefficients (lift, drag, moment) is the planform area. Your picture shows only one wing, while the reference area S would also take into account the fuselage and the other wing: orange times twice blue and then some.
In its most generic case, the equation for wing area S is $$\int_{-b/2}^{b/2} c \cdot dy$$ with b = wing span = distance between the wing tips, c = local wing chord (orange line), y is the coordinate in direction $Y_m$.

For clarity in reference, the picture above from my old uni book by prof. Gerlach on stability & control, which shows how the definition works when sweepback and dihedral angles must be accounted for. Note how the wing/fuselage intersection is defined for the reference wing area, by extending the local leading/trailing edges of the wings.


text:
How should the crew callout 1,000 ft. prior to assigned altitude?
When climbing or descending in a multi-crew cockpit, most SOP's require a verbal callout at 1,000 ft. prior to the assigned altitude.
I typically hear different callouts by different people:

"One to go"
"Six thousand for seven thousand"
"Six thousand climbing seven thousand"
"Six for seven"

Is this just a personal preference kind of thing or is there an actual safety benefit to one or the other?

I prefer "6,000 climbing 7,000" I don't like using "for" over the radio because it sounds like a number. (or the intercom)

@p1l0t: I do the same over the radio, but not sure how much that really matters in this situation.

Yeah it doesn't really matter over the intercom except it forms bad habits sometimes.

I'm tempted to call this one opinion based since I can only really say "one to go" is potentially less safe than the other three (from the perspective of verifying the assigned altitude, not arresting the altitude change).

@casey Yeah, that's the question. I'm hoping that there was a study or an FAA recommendation on it though!

I was trained to say "one thousand to [climb|descend]" or "five hundred to [MDA|threshold|etc]", but I haven't seen a study on it... favoriting this so I can! Over the radio, I always use "six thousand climbing seven thousand" because it's the most specific and fairly concise.

@p1l0t That's why, here in the South, we have evolved to use the word "fer" as in "Six thousand fer seven thousand." Works perfectly with no confusion! As for the question at hand, I prefer "One kilofoot." Alternatively, "one mille."

I interpret the question as a call out in a multicrew environment, not over the radio.

Still can be confusing when talking about numbers and forms bad habits anyways. Saying 2,000 for 5,000 or something just bothers me.

Albeit more difficult to misinterpret, "One 'to' go", suffers the same possibility for sounding like a number as "for".

As Ralgha stated, this kind of callout would be included in the SOP. A couple I found in searching were the NBAA (same in their helicopter procedures by the way) and Angel Flight NE, as well as some generic CRM guides. The FAA Instrument Procedures Handbook also mentions altitude callouts, suggesting something like "two to go" and "one to go" (in thousands).
The NBAA includes this specific phrasing:

The callout is to include the altitude vacating and the assigned
  altitude i.e. “Six thousand for seven thousand” or “Flight level
  three-zero-zero for two-niner-zero”. After the PF makes this call, the
  PNF will verify and validate the call by stating “check”.

This Flight Safety Digest mentions that USAir used to use the "two to go" and "one to go" phrasing, but opted to change this for the same that the NBAA uses. The new policy has the pilot flying make this call, since they are in control of the climb, and makes it more clear what the final altitude will be.
The Flight Safety Foundation went in to a bit more detail about the "why" behind the callout. They mentioned that in their observations, the callout was sometimes not done due to workload of other more important things going on in approach/descent. They explain the benefit:

This is not to suggest that the 1,000-ft callout is trivial. On the
  contrary, it ensures that both pilots concur about the altitude
  target, directs the attention of a flying pilot who might be
  distracted back to the impending level-off and draws both pilots’
  attention to what the autopilot is supposed to be doing.

Another resource that suggests the important is a report of an aircraft overshooting its altitude by 600 feet because the callout was missed. This report is in the ASRS database, report number 317750.
I also found it referenced in this book about communication. They didn't really comment on it specifically, but used it as an example of how communication is critical. Specifically, the more exactly the terminology is defined, the more clear the communication will be.

As far as I know, there are no published FAA guidelines on it.  An operator develops their SOP, and the FAA approves it, so whatever the SOP says is approved by the FAA.
Ours is, "one thousand to go," by the pilot flying, and then, "nine thousand for ten thousand," responded by the pilot  monitoring.

While I don't fly in a plane requiring more than one person, when there happens to be two pilots, such as one flying safety so the other can fly under the hood, I prefer the "1,000 feet remaining" or "1,000 remaining" call out.  I don't really have some scientific reason for it, but it just seems more logical to me and easier to interpret.

Our SOPs require Altitude leaving (climbing/descending ) to assigned altitude.  This verifies again that the proper altitude is set and correct.  One to go has too many meanings.  100/10000/1 mile/ one leg/ one minute/??  It’s ambiguous and has too many casual meanings and doesn’t verify altitude set. Easy to ignor the verification.

If the only time the SOP uses the callout "one to go" is in reference to altitude, it's hardly ambiguous or confusable for one mile/minute/etc.  That said, driving the crews to reference the altimeter rather than enforcing a Pavlovian responseto the alerter horn probably IS a good thing.  Welcome to Av.SE!


text:
Could COVID19 be considered as an ICAO abnormal or emergency situation?
Could COVID19 be considered as an abnormal or emergency situation as defined by ICAO:

Abnormal Situation:
A condition concerning the safety of an aircraft or other vehicle, or of some person on board or within  sight, but which does not require immediate assistance.
Emergency Situation:
A condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.

Added both ICAO definitions of "Abnormal" and "Emergency" situations .

Thoses definitions applies only to one aircraft. Do you mean "could a covid19 case discovered among a passenger while still in flight be considered as an abnormal or emergency situation?" In that case, you should update your question to more clearer.

Recently happened multiple times the aircraft ferrying passengers to the home country while some of these passengers are knowingly sick with COVID-19, and other passengers probably might be. But I am not sure if you are asking about this or about the whole industry.

No. An abnormal/emergency situation is a situation that applies to a single aircraft at a specific time. COVID-19 does not (generally) pose a specific risk to a specific aircraft. Obviously the entire aviation industry is in very bad shape at the moment, but that is not what we mean when we talk about an "emergency" in aviation.

Thanks exactly what I meant , "Emergency" is used in an ambiguous way .

@Ali no, there is nothing ambiguous about the way the word is used. Context matters


text:
Can I get training in the USA with a PPL from another country, but without converting it?
My friend's in South Korea, and he would like to get training in the USA soon. He currently has a PPL in Korea and it is still valid.
I'm sure that he can start an IFR course in the USA and another country's PPL could be converted to an FAA PPL license.
First question:
I'm wondering if he can get flight training or just time-building with a non-converted license, i.e. just his Korean PPL?
Can he fly just with a CFI just as dual time? I'm sure that he cannot log PIC time if he doesn't have an FAA PPL. However, he would like to be familiar with USA PPL flying first before starting the IFR training.
Second question:
He will apply to the TSA first for time-building, but I'm not sure which category should be approved. There are category 3 (instrument rating) and category 1 or 2 (training event). Which one should he do?

Part 1: Can a foreigner with a PPL in another country's system receive training in the US?
All other normal prerequisites such as ASFP, medicals, etc. aside, yes, a foreign license holder can receive training in the U.S.
Remember: You do not need a pilot certificate to receive training, but you would need one to solo or take a check ride (such as a student certificate).
If a person holds a foreign certificate, I believe they have to be flying an aircraft registered in their home nation in order to qualify to solo or otherwise act as PIC in a foreign country without obtaining a local certificate.
So, receive training?  Sure.  Solo/PIC/check-ride in an N-registered? No.
If what you mean to ask is "can he obtain a new cert/rating without converting".  No because the FAA does not issue South Korean pilot certificates, so that would not be possible.  The dual training received in the U.S. would certainly be able to be logged and count as valid training back home.
Part 2: ASFP Training Categories
From the flightschoolcandidates.gov FAQ section:

What category do I qualify for?
  Candidates seeking flight training are separated into one of four
  different categories. A brief explanation of the categories follows:
Category 1 - Candidates who seek flight training in the operation of
  aircraft with a maximum certificated takeoff weight (MTOW) greater
  than 12,500 pounds, but who do not fall into Category 2.
Category 2 - Candidates who seek flight training in the operation of
  aircraft with a maximum certificated takeoff weight (MTOW) greater
  than 12,500 pounds, and who:
Are employed by a foreign air carrier that operates under 14 CFR part
  1546; Have unescorted access authority to a secured area of an airport
  under U.S.C 44936(a)(1)(A)(ii),49 CFR 1542.229; Are a flight crew
  member who has successfully completed a criminal history records check
  in accordance with 49 CFR 1544.230; or Hold an airman's certificate
  that is recognized by the FAA or appropriate US military agency, with
  a type rating for a multi-engine aircraft that has a certificated
  takeoff weight of 12,500 pounds or more.
Category 3 - Candidates who seek flight training in the operation of
  aircraft with a maximum certificated takeoff weight of 12,500 pounds
  or less for the following training events:
Single Engine Land (SEL)
  -- Includes initial Airman's Certificate, including a private, recreational, or sport pilot certificate. If a private and/or
  commercial license is the candidate's initial FAA license, it is
  considered an initial airman's certificate and is not exempt.
  Instrument Rating (IR) Multi Engine Land (MEL) Each of these training
  events requires a separate training request. Clarification regarding
  Category 3 exemptions is covered in the next Frequently Asked
  Question.
Category 4 - Candidates who seek recurrent training in the operation
  of aircraft with a maximum certificated takeoff weight (MTOW) greater
  than 12,500 pounds, and are current and qualified on the aircraft for
  which they are requesting training. These training requests are
  submitted by the flight training providers -- Candidates need to
  "Select Providers for Recurrent Training" from the AFSP Candidate
  account. If you have further questions regarding your category, please
  contact your Flight Training Provider.

If he wants training intended for an instrument rating in an aircraft under 12,500 Lbs MTOW then category 3.

Except perhaps for a US government blacklist of terrorist countries, the US honors licenses of other countries (foreign GA pilots enter US airspace daily i.e. Russia, Canada, Bahamas & Mexico). If your friends South Korea PPL is current, and he can speak and understand English, also has no known medical condition that would disqualify him, within the US he may act as PIC of any aircraft he is rated for.
Foreign pilots must pass background checks before they are allowed to receive training. There are certain other requirements and a few exceptions (like ground training). See this AOPA article for all the specifics.
The foreign pilot prerequisites before training can start are:

Verify that you have an appropriate visa
Notify the flight school that you want to begin taking flight training
Ensure that you have a valid email address
Create a login account at TSA’s AFSP website
Apply for training on TSA's AFSP website
Wait for the flight school to acknowledge your training request
Pay the nonrefundable $130 processing fee per instructions emailed to you
Look for a “Preliminary Approval” email from TSA.
Submit fingerprints to TSA per the instructions emailed to you
Wait for TSA to notify you and the flight school of its decision
Once you have received TSA approval, START FLIGHT TRAINING!
On your first day of training, the flight school must take your picture

First, to act as PIC of a US aircraft in the USA, you must have an FAA pilot's certificate. Assuming that a South Korean PPL is ICAO-compliant then the easiest route for your friend is to 'convert' his PPL by requesting a foreign-based FAA private certificate. See this question for more information, but note the disadvantages mentioned in this answer.
Second, time-building using only a South Korean PPL really depends on what South Korea allows. It might be possible for your friend to rent an aircraft and hire a US commercial pilot to act as PIC while your friend flies it. In theory the FAA wouldn't care because your friend isn't acting as PIC, and the TSA wouldn't care because it isn't flight training. So if South Korea allows logging time flown abroad, then your friend could log it as 'Korean time'.
But I don't know if that's really practical, and your friend would have to pay the same money (or even more) that he would pay for instruction. It would be cheaper to just convert his PPL, do some ground school and a couple of orientation flights to learn about flying in the US, and then rent only the aircraft.
Finally, for instrument rating training your friend must have TSA approval. Instrument students are usually category 3, which is for aircraft less than 12,500lbs MTOW.
Overall, I'd say the simplest thing is to get both an FAA foreign-based private certificate (i.e. 'convert' the South Korean one) and TSA approval for instrument training. Then your friend has complete flexibility to fly whatever solo and training time he needs.

This would depend on the FAA certificate the pilot is seeking. Anyone is allowed to receive training toward any FAA certificate/rating with or without an FAA Pilot certificate. Still, they will need to have an FAA certificate to fly solo or test for an FAA certificate/rating (unless a non-FAA Commercial pilot is seeking an FAA ATP).
For a non-US citizen seeking training for an FAA Private/Instrument/Multi or First FAA issued certificate, they will need TSA FSTP approval (Catagory 3). But if a pilot applies for an FAA Restricted Private Pilot (Foreign Based) certificate per 61.75 they don't need TSA approval (the FAA/TSA background check is run when the certificate is issued).
TSA approval is not required for the training a pilot receives other than for the above certificates/ratings. A few examples of training not requiring TSA approval would be a Flight Review, instrument proficiency check, familiarization training, or recurrent training in aircraft under 12,500lb.
So if the pilot applies for and is issued an FAA Restricted Private Pilot (Foreign Based), and then receives a flight review from an FAA Instructor. This pilot could then accomplish Time Building by renting an aircraft in the US.
But if the pilot accomplishes the time building with an FAA instructor and desires to count this training toward the FAA Private/Instrument/Multi or First FAA-issued certificate then the TSA FSTP category 3 approval would be required.
Additionally, if the pilot is receiving training from a non-FAA instructor the training may count toward an FAA Certificate/Rating (Reference: 61.41)
So for the FAA instrument rating, the training time required to have been received from an FAA Flight Instructor-Instrument Airplane would be 15 hours of flight training and some amount for ground training to cover all the required areas: (Reference 61.65)

(d) Aeronautical experience for the instrument-airplane rating.  A
person who applies for an instrument-airplane rating must have logged:
(1) Except as provided in paragraph (g) of this section, 50 hours of
cross-country flight time as pilot in command, of which 10 hours must
have been in an airplane; and
(2) Forty hours of actual or simulated instrument time in the areas of
operation listed in paragraph (c) of this section, of which 15 hours
must have been received from an authorized instructor who holds an
instrument-airplane rating, and the instrument time includes:
(i) Three hours of instrument flight training from an authorized
instructor in an airplane that is appropriate to the
instrument-airplane rating within 2 calendar months before the date of
the practical test; and
(ii) Instrument flight training on cross country flight procedures,
including one cross country flight in an airplane with an authorized
instructor, that is performed under instrument flight rules, when a
flight plan has been filed with an air traffic control facility, and
that involves—
(A) A flight of 250 nautical miles along airways or by directed
routing from an air traffic control facility;
(B) An instrument approach at each airport; and
(C) Three different kinds of approaches with the use of navigation
systems.

For the FAA Commercial Pilot certificate, the training time required to have been received from an FAA Flight Instructor Airplane would be 3 hours of flight training and some amount for ground training to cover all the required areas: (Reference 61.125/61.129)

(a) For an airplane single-engine rating.  Except as provided in
paragraph (i) of this section, a person who applies for a commercial
pilot certificate with an airplane category and single-engine class
rating must log at least 250 hours of flight time as a pilot that
consists of at least:
(1) 100 hours in powered aircraft, of which 50 hours must be in
airplanes.
(2) 100 hours of pilot-in-command flight time, which includes at
least—
(i) 50 hours in airplanes; and
(ii) 50 hours in cross-country flight of which at least 10 hours must
be in airplanes.
(3) 20 hours of training on the areas of operation listed in §
61.127(b)(1) of this part that includes at least—
(i) Ten hours of instrument training using a view-limiting device
including attitude instrument flying, partial panel skills, recovery
from unusual flight attitudes, and intercepting and tracking
navigational systems. Five hours of the 10 hours required on
instrument training must be in a single engine airplane;
(ii) 10 hours of training in a complex airplane, a turbine-powered
airplane, or a technically advanced airplane (TAA) that meets the
requirements of paragraph (j) of this section, or any combination
thereof. The airplane must be appropriate to land or sea for the
rating sought;
(iii) One 2-hour cross country flight in a single engine airplane in
daytime conditions that consists of a total straight-line distance of
more than 100 nautical miles from the original point of departure;
(iv) One 2-hour cross country flight in a single engine airplane in
nighttime conditions that consists of a total straight-line distance
of more than 100 nautical miles from the original point of departure;
and
(v) Three hours in a single-engine airplane with an authorized
instructor in preparation for the practical test within the
preceding 2 calendar months from the month of the test.
(4) Ten hours of solo flight time in a single engine airplane or 10
hours of flight time performing the duties of pilot in command in a
single engine airplane with an authorized instructor on board (either
of which may be credited towards the flight time requirement under
paragraph (a)(2) of this section), on the areas of operation listed
under § 61.127(b)(1) that include—
(i) One cross-country flight of not less than 300 nautical miles total
distance, with landings at a minimum of three points, one of which is
a straight-line distance of at least 250 nautical miles from the
original departure point. However, if this requirement is being met in
Hawaii, the longest segment need only have a straight-line distance of
at least 150 nautical miles; and
(ii) 5 hours in night VFR conditions with 10 takeoffs and 10 landings
(with each landing involving a flight in the traffic pattern) at an
airport with an operating control tower.

So for the Commercial they only need to accomplish a minimum of 3 hours of training from an FAA instructor (reference: 61.41) (the remaining time can be from a non-FAA instructor assuming the logged training (per 61.51) is correct. The logged training must include the


text:
What do these symbols indicate on the ATC display?
I just got found ATC controller display on the internet and could you tell me what are these symbols indicating? For example the ones I highlighted in red?

You might want to refer to the manual: https://www.ivao.aero/softdev/ivac.asp#doc

The white squares are aircraft - it's course is indicated by a line. Then there is a datablock showing Callsign "FWA170C", its altitude F053 = Flighlevel 53 = 5300ft, climbing to an assigned flightlevel 190 = 19000ft.
A = altitude - below a defined transition level
F = flightlevel (steps of 100ft) - above a defined transition altitude
Transition altitude and level depend on the location and weather.
Then you have the airports, navaids, nav-fixes and the control zones showing.

I believe the C in FWA170 C (there is a space, I believe) might be transponder mode being Mode-C.

@tsg thanks and how could the ATC know the aircraft is climbing to FL190? Is that reported by the aircraft?

@SentryRaven C is an indication of the current sector ("Center").

@VvV Please get in touch with the IVAO training department. They have plenty of helpful resources.

@J. Hougaard some Aircraft has a ident end with "T", what does that mean?

@VvV http://forum.ivao.aero/

To add to the answer... The long white line between the FWA170C (data block) and the small white square (airplane) is just there to connect the data legend to an airplane... following the line from the airplane let's you find the data associated with that aircraft.

ATC knows they're climbing to FL190 because that's what ATC told them to do...

@Jan, yes, and the short line coming out of the centre of the square is the current movement vector. I think its length is 5 min of flight, but I don't remember exactly now.


text:
When an emergency happens, why do pilots need to stay on the current ATC frequency?
Per the below order, I need to stay with the initial ATC frequency during an emergency, what is the benefit to do so? What if the aircraft entered into the airspace controlled by anther ATC or the aircraft is no longer within line of sight?
FAA Order J7110.65V, §2, ¶10-2-2:

Although 121.5 MHz and 243.0 MHz are emergency frequencies, it might
  be best to keep the aircraft on the initial contact frequency. Change
  frequencies only when there is a valid reason.

Related, possible duplicate: On an engine failure, can I contact the tower instead of 121.5? (Full disclosure: The accepted answer on that one is my own.)

[the aircraft entered into the airspace controlled by anther ATC] is a valid reason.

This doesn't mean you won't change frequencies if you move into another controller's sector or there's another reason for a handoff, such as moving from Center to Approach or Approach to Tower; it's to say that if you're already talking to ATC at all (often optional for VFR), then you should report your emergency there rather than switching to 121.5.
121.5 is monitored at every ATC facility, so if you have an emergency and don't know the right frequency to use, you can always go there and someone will answer you--and presumably give you a better frequency as soon as they've figured out where you are. If you already know the right frequency, though, then it's better for everyone (including you) to use that instead.

A pilot having an emergency is told to aviate, navigate, and communicate. Presumably the pilot has his/her hands full, and an unwarranted frequency change adds to the complexity.
If you were flying through my sector at FL370 and had to descend, I'd assign you FL350 and switch you to the next controller, who would assign FL240 and switch you to the next controller, who would assign you 110 and switch you to approach control.
If you were an emergency, I would likely coordinate with the underlying sectors, assign you 110, and switch you to approach control. Eliminating two frequency changes gives you the pilot more time to work the problem, hopefully leading to a successful outcome.

And removing a frequency change, removes two possible mistakes, in a situation where they are more likely, and potentially more costly.

Here is another possibility, almost the inverse of @StephenS's suggestion. (Note that Stephen's suggestion is very much correct as well; if you are already in contact with ATC on a "normal" frequency, there is no reason to switch to 121.5 for the purpose of declaring emergency.)
Perhaps the paragraph should be interpreted like this:

Although 121.5 MHz and 243.0 MHz are emergency frequencies [and therefore should not be used except for initial contact, so as to keep the frequency clear for other potential emergencies], it might be best to keep the aircraft on the initial contact frequency. Change frequencies [away from 121.5/243.0] only when there is a valid reason.

In other words, the paragraph is telling the controller that the pilot remaining on 121.5/243.0 could be a safer option than having them change to the controller's normal non-emergency frequency. This could be the case, for example, if a VFR pilot is in IMC; a non-IFR-rated pilot will have limited experience flying without reference to the horizon and having the pilot move their head around—which would be necessary to adjust the radio frequency—could lead to disorientation and a loss of control.

As abelenky said in a comment, if you're talking to one controller, you already know you're in contact with them. If you call a new controller however, you have to establish contact, and they have to positively ID you.

I'm still a ATPL student and this is how i have understood from Airlaw.
ATC will know on their display if you're out of their sight or not (they'll do all they can to not give you more pressure), and they'll make direct call to your next destination ATC when you are in a distress.
Today we also have Data link system and the next ATC can also recive it on their screen and alert to the Rescue team in advance. And if you are out of sight you just need to switch frequency to your next ATC or the frequency you are on could still help ypu out  or if you can switching to your next ATC and mention your Flight number exm: Birminghead-125 distress "postion" and "altitude" and your currently "situation" to update if the situation has escalated or not. That's how they do all the time, keep updating the situation is the key, and also aircraft around you sight can also forward your message etc.
Edit: i forgot to explain your scenario about benefits with not changing frequency is, if you have COM problems or COM equipment in aircraft doesn't work properly, or when you are transferring in blind so you don't need to switch to anyone.


text:
On an ADS-B In display, do the pilots see the squawk code or call sign for other aircraft?
If I understand correctly, with a ADS-B In equipment or TCAS equipment on board, the pilots could see the surrounding traffic with its assigned squawk code like "4653", is that right? But could the pilots know the aircraft identification like "UA858"?

ADS-B is based on mode-S, NOT on squawk codes. Only mode-A transponders use squawk codes.

ADS-B is part of a system that compromises a number of different things. ADS-B (In) means that your aircraft is capable of receiving the ADS-B data, FIS-B and TIS-B as well. FIS-B broadcasts information such as weather. TIS-B is a traffic broadcast system.
ADS-B includes a number of messages (about 27 different messages), including:

Aircraft Identification
Surface Position
Airborne Position (Barometric)
Airborne Velocity
Airborne Position (GNSS)
Aircraft Status
Target State/Status
Aircraft Operation Status

Each message packet holds 56-bits of data. The aircraft identification message includes the aircraft type, category, and callsign.
Now, to a pilot, here is basically what you would see:

Source: NexAirAvionics.com
You can see that some targets include the flight number (JBU1075) and clicking on a target (this is in Garmin Pilot) will give a little more information on the target:

Source: Garmin.com
What the pilots don't see is the squawk code, this information is not useful to pilots because they don't really care about that for other aircraft. The only real use of ADS-B target information is to understand where other aircraft are. If a pilot needed to call up another pilot, they would do so by tail number or flight number. ATC Controllers and other pilots will never call a pilot by squawk code.
Why? Because the squawk code changes potentially many times during a flight. Let's take for example a simple flight from KGRB to KMNM (a flight I'm familiar with):
On the ground at KGRB, when telling ATC about my destination they will assign me a squawk code, for example 2044. After departing they will call me up "1205C turn on course, altitude your discretion" and I will acknowledge. About 30 miles out (if I didn't get flight following) they will say "1205C squawk VFR, frequency change approved". Now my squawk code changes to 1200. I might then pick up Minneapolis Center if I want to change my destination and they will assign me a different squawk code.
The one thing that remains constant there is my tail number. I know to listen for (and have mentally trained a "listen up" trigger when I hear) "1205-charlie" or "05-charlie" on the radio. ATC has been talking to specific aircraft using the tail number/flight number since the ATC system was born. Squawk codes match aircraft with targets on the radar (because before ADS-B all that was transmitted was the squawk code). This information is only useful to ATC and not other pilots.
Now if I'm cruising around and I see a target on my ADS-B that is headed in the same general direction I am, and I'm not talking to ATC, I can try calling that target using the tail number/flight number directly (this is done on the air-to-air band or a local frequency, don't broadcast on 121.5 or you'll hear "guaaaaarrrrrrd" from some obnoxious pilot).
Either way ADS-B is useful for traffic avoidance. I don't really need to know where that airplane is going, why it's going there, etc. I just need to know what it is (big jets can cause wake turbulence issues for smaller aircraft), which way it's heading, and how fast it's going there.

The display depends on the equipment used.  For example, the Avidyne 540 Pilot Guide says this:

For Mode-S equipped aircraft, the aircraft ID (e.g. tail number, call
  sign, etc) may also be displayed adjacent to the traffic symbol.

Normally I see a solid white diamond with a dotted line leading away to indicate direction, + or - how many hundreds of feet above or below me that traffic is, and an arrow if the traffic is climbing or descending. That would be plain ADS-B Out from other small planes like mine.
The diamond may also be hollow, blue, a yellow circle, a white arrow, a yellow arrow, all meaning different things.  I'm not going to copy all descriptions from the Avidyne manual. One can read more here  https://www.avidyne.com/support/doctype.asp?doctype=pilot%27s+guides&list=doc  Scroll down to IFD-xxx, download it, and then page 3-39 for Traffic Display in the April 2019 release.
If the traffic is close enough, I'll also get a "traffic, 1000 high, 2 miles" kind of aural message over the intercom. Typically when other traffic are nearby in the pattern, or perhaps when crossing paths at intersecting angles (like me at VFR and IFR traffic going by 500 feet above or below).


text:
Where can I find flight plan records for a private flight in the 1970s?
How would I find a record of a private plane flight from Canada to the US in the early to mid 1970's? I am researching records of my father.

Depends on the type of aircraft, but if any records have managed to not be destroyed after 48 years, I would suggest you start by talking to the owner/operator of the departure and arrival airfields. US Customs might have a record of a customs inspection if there was one, but you'd have to file a FOIA request and be prepared to wait for an answer which may not be the one you want to hear.


text:
How can I make a cheap and simple straight and level recovery device for an RC airplane?
I am a new RC pilot and I crash my RC aircraft a lot. I was thinking of a cool device which would be like training wheels for a plane: a device to return the plane to straight and level flight at the touch of a button.
I understand that I can do that with an autopilot but they are over \$100 and have more features than I need. I was thinking of something like a $2 6DOF MPU-6050 3 Axis Gyro With Accelerometer Sensor Module and an Arduino.
I'm thinking the Arduino would intercept the servo controls and when activated, it would use the data from the gyro to correct pitch and roll.
Does this sound feasible?

Are you familiar with Arduplane/Arducopter? Sounds like what you want to do already exists.

This question isn't really on topic on this forum.

Depending on your experiences and abilities  with the arduino and  the other hardware, it definitely sounds feasible. Be careful to not introduce too high AoA when programming the recovering maneuver.

Hobbyzone SAFE technology may interest you, and it can be had for a reasonable budget.
The RC groups internet site may be good as well.
I include this answer on Aviation Stack Exchange to hold out the possibility that such
devices may be considered for full scale aircraft as well in the future.  They are able to almost instantaneously recover an aircraft from out of control to wings level with a push
of a button.  Videos of this function are impressive.


text:
Why did the DC-3 have wing sweep?
When looking at the silhouette of a DC-3, one can clearly see that the wings are swept. This design is from 1935, before the high-speed research work on wing sweep was ever implemented, and the DC-3 was not very fast anyhow.
Leading edge sweep was 14 deg and trailing edge 0 deg, so the quarter chord wing sweep was 10.5 deg, not inconsiderable, and causing some of the ugly stall behaviour associated with wing sweep. A 1938 NACA video of a DC-3 stalling here, a bit lengthy but it illustrates the tip stalling first, then the aileron.
None of the benefits and some of the drawbacks - why did the DC-3 have wing sweep?
Image Source

Notice how the trailing edge is at a right angle to the fuselage, which simplifies the aileron design. My first instinct says the design was settled on a tapered wing and they had to choose which wing edge would be at a right angle to the fuselage, and went with the trailing one. Overall that amount of sweep is neglegible.

Many other aircraft had tapered wings at the time. The DC-3 had 14 deg leading edge, 0 deg trailing, resulting in 10.5 deg at quarter chord line. The Fokker 100 has 15 deg at quarter chord, flies at Mach 0.77.

The wing was swept to move the center of lift back, closer to the center of gravity. When the design of the DC-1 was advanced to a point where the center of gravity became clear, a redesign of the wing-fuselage intersection would had been more effort, so the outer wing panels were swept back a little.
From this website by Jeff Lucker:

the unique and characteristic swept wing of the DC-3 was an attempt to
  keep the wing in its original location as the center of gravity moved
  toward the rear.

When Douglas produced new variants (DC-2 and DC-2 sleeper, a.k.a. DC-3), they kept the proven layout.

Why did they do that, instead of putting a plug in front of the wing, I wonder.

@Koyovis: At those times the fuselage was aerodynamically shaped and not meant to be extended. Every bulkhead was different (a tradition which became a liability for the Constellation later). Only with pressurization and the rationalization in production during the war did the preference shift to cylindrical fuselages.


text:
What are the requirements for a small jet type rating in the UK?
Just out of curiosity, how would one go about getting a type rating for a small, twoish seater jet (for example the Viperjet Mk2) in the UK?
I've had a brief look but cannot find anywhere that offers such a thing.

As far as I know, you can't get a type rating for a homebuilt jet in the UK. You can do so in the US but you have to find an instructor and examiner rated by the FAA to do that. I would suggest you talk to the manufacturer of the kit about this. They are the most likely to know the answer, but I would not hold out much hope. The UK CAA has historically demonstrated they are not too keen to let people fly homebuilt jets.

Okay, thanks for the info

And for pretty good reason

@Cloud And what would that be? That they don't think UK pilots are good enough to build and fly homebuilt jets? That must be why Ireland allows it.

I would say GA hobbyist pilots in general are, on average, not experienced enough to fly jets.

And that is exactly what I would expect a clueless CAA bean-counter to say. Which is why I tell people that if they want to build and fly a homebuilt jet, move to Ireland. Better booze too.


text:
How does wing aspect ratio influence lift and drag?
I know simmilar questions are all over the internet (and this website too), but there's one thing that i just cannot comprehend and none of the articles i read could explain it properly.
According to wikipedia the coeficient of drag depends on the aspect ratio by the following way:
$$
C_D=C_{D0}+ \frac{(C_L)^2}{\pi e AR}
$$
But when i search why does it have this negative impact, i read things like "high AR creates stronger vortices at the tip of wings, which reduce lift".
But reducing lift reduces the numerator of the expression, so that should in fact REDUCE the drag. Does the $C_L$ somehow also depend on $AR$? If yes, is the dependancy also linear or less?
And another thing that blows my mind are statements, that wing which spans the  whole width of a wind tunel produces no induced drag, because it has no wing tips. Are you telling me that the induced drag is only caused by wing tips?
From the pictures at How complete is our understanding of lift? one would think that drag is natural part of lift and is caused by the airfoil even in 2D without considering wingtips.
So to sum up, can you please properly explain me:

How does the aspect ratio influence induced drag, why is the AR in
denominator
How does it influence lift
What really happens at the wing tips
What happens in the wind tunel on a wing that spans its
whole width

low aspect would create stronger vortices for a given wing area.  There is good and bad info here, but in time in sorts out.  My understanding is induced drag is caused by raising angle of attack to create lift.  This also increases drag, so induced drag is added to CDo.   Hope info here helps clear it up.

Your suspicion is leading you in the right direction. A lot of what you read on the Internet about induced drag is at least misleading if not blatantly wrong. But it is repeated by sloppy authors who don't ask the right questions.
If you have read stuff like "high AR creates stronger vortices at the tip of wings, which reduce lift", you hit such a completely wrong statement. The misleading version of it is high AR creates weaker vortices. For a hopefully better explanation, please turn to this answer.
Now to your questions:

How does the aspect ratio influence induced drag, why is the AR in denominator?

The surprising answer: It is the lift per unit of span and the inverse of the square of flight speed which determine induced drag. Another influence is the distribution of lift over span, but aspect ratio does not play a role. See this excellent answer by @DeltaLima for an explanation, or this one for a longer one.

How does it influence lift?

Indirectly, by increasing the lift curve slope.

What really happens at the wing tips?

On the top side, suction turns the airflow inwards so the air gets a rotational component and some of it flows around the tip from bottom to top. Due to the small wingtip curvature, this vortex has low pressure at its core (and lets air condense early), but is overall weak and rather insignificant. The real trailing vortex results from a rollup of the wake, but even this does not create drag.

What happens in the wind tunnel on a wing that spans its whole width?

Please read the answers to this question.

I have complementary question. You wrote "A higher wingspan allows to capture more air for lift creation". Which means at lower AR, more air is "leaking" at the tips and not participating in creating lift. But it still participates in creating drag, why? The linked answers just play with transformation of the equation to show the relations mathematically. But i'm searching more for an aswer WHY the equation looks as it does. Let's say i want to keep total wing area same. Why does the low AR negatively impact drag, why the leaked air doesn't create lift, but still creates induced drag?

Also, when they break up the equation and show that induced drag in fact depend on span, i wonder what happens if i increase the wing area chord-wise. Why does span-wise increase of area reduce induced drag, but chord-wise increase of area doesn't?

@Youda008: There is no air which creates only lift or drag. All air creates an aerodynamic force which is per definition split into lift and drag components.Lower AR means more deflection of less air for the same lift force, and more deflection means more backward tilt of the aero force. The backward-facing component of this force (without friction-related components) is induced drag.

@Youda008: More chord only means that the same air can be influenced over a longer flow path. This means more lift, but also more backward tilt. More span can influence more air which needs less deflection to produce the same lift. Less deflection = less backward tilt = lower induced drag. If that does not help, I am out of my wits to explain it.

Ohhh, THAT was the missing puzzle. So widening the wing chord-wise basically equals to increasing AoA or deploying flaps?

@Youda008: I prefer to say "lengthening the wing chord-wise", but yes, it is roughly equivalent.

Well, let's pick those pieces of your mind up and get this sorted out.
CDtotal = CDo + Clsquared/pi x e x AR
Reading the NASA website the following is deduced.

CDo is the zero lift drag, essentially everything except what produces lift.  For simplicity they only consider induced drag from the wing, even though fuselage and tail drag will change with AoA.
The rest is induced drag from the wing.  Drag increases with increase in Cl squared, more drag is a product of more lift.  Drag decreases with increased AR.  Look at a sailplane.

Why does increased Aspect ratio make a wing more efficient (more lift per unit of thrust) for the same area? Simply because there is a pressure "leak" at the end of the wing, making it produce less lift per area than the rest of the wing.  The smaller the wingtip chord is relative to the area of the wing, the smaller the percentage of this lift loss is relative to total lift.  There for, a lower aspect wing of equal area needs a higher AOA to produce the same lift, producing more drag (and more energetic vorticies).  The plane must add more power to maintain velocity, hence the higher aspect wing is more efficient.  This is how aspect ratio is related to drag, and why it's value is in the denominator.
Remember, vortices are the RESULT of lift creation. They are literally an expression of how much energy the passing wing has imparted to the air.  Lift is how efficiently the air moving energy was used.  Drag = Thrust!
Finally, a wing that spans the entire wind tunnel would be useful to focus on the airfoil shape performance at various AoA and to provide an "infinite" AR data point to compare with a range of AR to be tested.


text:
How do I optimize a turbofan engine to fly lower and slower instead of higher and faster?
I was reading something about someone who was doing this and I'm intrigued, but I have no idea what you actually do to optimize the engine to be better at lower altitudes and lower speeds?

Why do you want to do that? That is, what do you want to achieve? Or in other words, what is not satisfactory in the existing solution - a turboprop?

He must know a good mechanic.  Those Cyclone radials were pretty good on gas.

Welcome to the site @JustinHolderman, I think the answer may be "you don't".

As a summary, the engine generates thrust by changing the impulse (momentum) of the air. Please refer to this: NASA Genera Thrust Equation
So to optimize aircraft performance, the main options you can play with are that depend on your aircraft operation:
V0: flight speed
p0, r0: inlet pressure and density (function of altitude)
And the variables that depend on your engine design:
Ve, pe, re: exhaust velocity pressure and density (depend on the thermal cycle and actual operating condition of the engine)
A0, Ae: Inlet and nozzle areas (engine geometry, sizes derived from the design point)
Now what you need to see is that there is no single best option, aircraft design is an optimization problem, eg finding compromises. Flying lower and slower is actually an attractive option for greener aircraft, as it enables technologies that reduce drag, such as ultra high aspect ratio wings, laminar airfoils, boundary layer ingestion, electric propulsion and a few more.
Whether optimizing a turbofan for lower altitudes make sense can only be seen by comparing the wins against the losses, and it is hard to say absolute truths as it depends on many variables.
Now about the actual "how-to" optimize a gas turbine, essentially the key variable of the engine is the pressure ratio, PR, so how much it compresses the inlet air. For a given flight condition (thrust, speed and altitude) , there is an optimal pressure ratio. From that the number of compressor and turbine stages can be calculated (also if you need a multi-spool engine, do you use an intercooler, etc.), and also the geometrical dimensions of the engine. And this goes back to the comment I've made that it is a compromise. Building a very heavy, expensive, hard to maintain engine will not be a good choice as what you win by increased engine efficiency you will lose on the flight efficiency.
But suppose as an academic exercise you just want to alter an existing engine design to move its design point to a lower speed and altitude, then you need to change the design pressure ratio, and follow up with any internal changes (add/remove stages, change inlet, nozzle, etc.)


text:
How do I calculate FPNM for standard take-off minimums?
I have trouble understanding and calculating FPNM for standard take-off minimums.
As I understand, the formula is: (Ground speed)/60 * FPNM = FPM
Let's say the ODP says:

565' per NM to 2800, or 1000-3 with minimum climb of 370' per NM to
  2800, or 1500-3 for climb in visual conditions

With a Cessna 172S, could you let me know whether the aircraft is able to meet this requirement? Here is the rate of climb chart:

I suggest that you put some numbers into your formula and see if the result makes sense. If not, or if you don't understand the result, feel free to to add it to your question: a specific example would help us to understand where your problem is.

To convert Feet per minute to feet per nautical mile you must divide the climb rate by your ground speed.
$$
 \frac{625 FPM}{(79 KIAS / 60)} = 474 FPNM (nowind)
$$
Conversely, you can use the table in the back of the instrument procedures booklet.

Electronic version of TPPs are found at the bottom of this page
https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/

If your takeoff speed is 60 knots, and you're climbing at 500 feet per minute, then you are climbing at 500 feet per nautical mile,  yes? Extrapolate from there.

Not exactly. Feet per nautical mile is based on ground speed, not airspeed. And like the chart shows, fpnm varies with temperature and altitude.

@JScarry, Crossroads used "knots", not IAS.  (knots could also mean groundspeed and airspeed could be in MPH...)  Granted GS in knots wasn't specified, but neither were winds.  Given the simplicity clearly intended to make a point, it's pretty safe to assume a no-wind situation was meant here.

You don’t specify an altitude in your question, so let’s start with 2000' and the requirement to climb at 565' per NM and assume you have the airspeed at 73 kts. Then the performance table says
-20°   0°   20°    40°

760   695   555   560

So you meet the requirement for all temperatures up to almost 40°.
At 4000' you will make it at 20° but to see what the maximum temperature would be you need to interpolate.
-20°   0°   20°   ?°   40°

685   620   625  565  496

There are lots of ways to do the arithmetic, one way is with percentages.
The difference between 625 and 496 is 129. The difference between 625 and 565 is 60. So 565 is 60/129 (47%) of the way between the two. Our unknown temperature is also 47% of the way between 20° and 40°.
The difference in temperature is 20°. 47% of that difference is 9.4°. We went from left to right in calculating the percent difference so we do the same thing for temperature and add 9.4° to 20° to get 29.4°.
So as long as the temperature is less than 29.4° and you maintain an airspeed of 73 kts indicated, you can meet the climb gradient.
If you are at an altitude in between one on the table then you would need to do two interpolations. One at the lower altitude and one at the higher.
In the real world, and not on an FAA test, just use the next higher altitude and temperature.
Note that the table is based on pressure altitude, so set your altimeter to 29.92 to get the pressure altitude of the field.
Also note that if your airspeed is faster than in the table, you will be climbing too slow to make the numbers. This is also a no wind table. If you are climbing with a tail wind you will not meet the climb rate per nautical mile.
And finally, the climb can be to an altitude that is fairly high above the field elevation. If you look at the chart you will see that performance drops off with altitude. In your example if the field is sea level, then you will need to climb 2800' at 565 fpnm. Don’t use the sea level numbers since you are climbing through the 2000' level. Use the 2000' numbers to be safe.
My home field is at 212' MSL and one of the departures has a minimum climb of 320' per NM to 3000'. I’d use the 4000' row to decide whether I could make the departure.

The response from JScarry above is good, but I think the Cessna table provides values in Feet/Minute, so you still need to convert that FPM to Feet/Nautical Mile. Be careful.

Downvoted for confusing FPM and FPNM.  Fix and I’ll reverse because this is a good discussion of interpolation.


text:
What does the "visibility" number mean on an approach plate?
In the profile view in an approach plate, there is a visibility number next to the MDA or DA. What does it mean? Is it limitation to use the approach?

This question can be improved if you can include an image of the approach as an example.

If this question is about the US, the FAA's Chart User's Guide is a useful source for questions about approach plates or any other charts. It says:

This section gives the pilot the lowest altitude and visibility
  requirements for the approach. There are two types of landing
  minimums: Straight-in landing or Circling. Straight-in landing
  minimums are the MDA and visibility, or DH and visibility, required
  for a straight-in landing on a specified runway. Circling minimums are
  the MDA and visibility required for the circle-to-land maneuver.

And:

The visibility values are shown after the DA/DH or MDA. They are
  provided in statue [sic] miles or runway visual range (RVR). RVR is reported
  in hundreds of feet. If the visibility is in statute miles, there is
  an altitude number, hyphen, whole or fractional number, e.g. 530-1.
  This indicates 530 feet MSL and 1 statute mile of visibility. The RVR
  value is separated from the minimum altitude with a slash, e.g.,
  1540/24. This indicates 1540 feet MSL and RVR of 2400 feet.

The number is indeed a limitation: if the required visibility doesn't exist then the pilot can't use the approach. However, there are some differences in what that really means depending on the type of flight: an airline flight (part 121) can't even start an approach if the reported visibility is less than the approach requirements; a private flight (part 91) can start an approach but may not land if the flight visibility is below minimums. This question might be relevant.

That is typically a Runway Visibility Range (RVR) number expressing the runway visibility in thousands of feet.  The pilot will have to determine whether local weather conditions permit runway visibility at or greater than that published figure in order to successfully terminate the approach in a landing.  If it cannot be done, the flight crew must initiate a missed approach.


text:
How can I determine the relative bearing to an NDB using the G1000?
For using an NDB, a pilot should know to MH+RB=MH. But I have never seen a relative bearing displayed in the G1000. Could you let me know where to see the relative bearing in the G1000?

I don't have an answer, but I'm not sure why you would want to do this anyway. The G1000 can take you directly to an NDB in the same way that it can take you directly to any other waypoint. But having said that, my G1000 experience is in a C172 that doesn't have any ADF. In an aircraft that does have ADF, it may behave differently.

Are you asking for a digital readout of this information? The relative bearing is read from the HIS as the difference between the NDB needle and heading indicator.


text:
How to decide on a rate of climb or descent if no specific vertical speed is required?
Obviously, obstacle clearance, charts, atc instructions etc. can require a specific rate of climb or descent, or at least pose limits that are easily calculated. When no such restrictions apply, there appears to be a large amount of freedom in selecting a vertical speed for reaching a certain instructed altitude. Are there standard rates in common use? What (preferably quantifiable) factors help pilots decide how quickly they want to get to their desired altitude?

In general terms, the higher you are the better your fuel efficiency, so you will want to climb quickly and then stay as high as you can until you need to start your descent in order to land.  For jets, a common way to approximate your top of descent is by using the "3:1 rule" where you take your desired altitude loss in thousands of feet and multiply it by three.  For instance if you need to lose 35,000 ft. you would start down 105NM ($\frac{35000ft \times 3NM}{1000ft}$) prior to your destination, staying at altitude until then in order to save fuel.
As far as your rate, the first place I would check would be the AFM of the airplane that you are flying.  Smaller airplanes will have Vy (best rate of climb speed) that may be used for this purpose unless another speed is specified (to help with cooling, etc).  Larger aircraft often have specific climb and descent schedules (for example, climb at 250 KIAS until 10,000 ft then 300 KIAS until you transition to Mach 0.76) which the manufacturer recommends and provides performance information for.  Fly these speeds and take what vertical speed that you get.  Usually, flying by the book is the place to start unless you have a specific reason to do otherwise.
The AIM 4-4-10 says:

d. When ATC has not used the term “AT PILOT'S DISCRETION” nor imposed any climb or descent restrictions, pilots should initiate climb or descent promptly on acknowledgement of the clearance. Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached. If at anytime the pilot is unable to climb or descend at a rate of at least 500 feet a minute, advise ATC. If it is necessary to level off at an intermediate altitude during climb or descent, advise ATC, except when leveling off at 10,000 feet MSL on descent, or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area), when required for speed reduction.
NOTE- Leveling off at 10,000 feet MSL on descent or 2,500 feet above airport elevation (prior to entering a Class C or Class D surface area) to comply with 14 CFR Section 91.117 airspeed restrictions is commonplace. Controllers anticipate this action and plan accordingly. Leveling off at any other time on climb or descent may seriously affect air traffic handling by ATC. Consequently, it is imperative that pilots make every effort to fulfill the above expected actions to aid ATC in safely handling and expediting traffic.

It's also worth noting that in small (unpressurized) aircraft your ears will tell you if the climb or (more likely) descent rate is too fast for you: If you or your passengers need to keep popping your ears you should probably reduce your rate of altitude change a little, unless there are operational factors that prevent you from doing so.

@voretaq7: I agree!